1. Field of the Invention
The present invention relates to a vacuum heat-insulating panel used as a heat insulating material, in a gap formed between a molded product of metal thin plate and a resin molded product, on a wall surface which is required to be heat-insulated, for example, in a refrigerator, a cold reserving car or the like, and a method for producing such a vacuum heat-insulating panel.
2. Description of the Related Art
Recently, in order to protect terrestrial environment, restricted use regulation has been imposed on chlorofluorocarbons for the purpose of suppressing the ozone-layer depletion ratio, and even on hydrochlorofluorocarbons which can suppress the ozone-layer depletion ratio to 1/10 or less in comparison to chlorofluorocarbons. Accordingly, a heat insulating system capable of improving the heat insulating property without using any blowing agent and capable of recovering and recycling used materials will be required socially in the future.
Conventionally, in a heat insulator wall of a refrigerator, a cold reserving car or the like, rigid polyurethane foam is injected into a gap formed between an outer housing product molded of a metal thin-plate such as an iron plate, and an interior part of a resin-sheet molded product so as to fill the gap with the rigid polyurethane foam.
Although 1,1-dichloro-1-fluoroethane which is one of the hydrochlorofluorocarbons capable of providing excellent heat insulating property has been used as a blowing agent for rigid urethane foam which is a heat insulating material, use of hydrofluorocarbons or hydrocarbons the molecules of which contain no chlorine which causes ozone-layer depletion has been proposed in recent years. For example, JP-A-2-235982 discloses a method for producing rigid polyurethane foam in which hydrofluorocarbons such as 1,1,1,3,3-pentafluoropropane (hereinafter referred to as HFC-245fa), 1,1,1,4,4,4-hexafluorobutane (hereinafter referred to as HFC-356mff), etc. are used as a blowing agent, and JP-A-3-152160 discloses a method for producing rigid polyurethane foam in which hydrocarbons such as cyclopentane, etc. are used as a blowing agent. When such a kind of rigid polyurethane foam is applied to a refrigerator or the like, however, the heat insulating property is in a range from 17 to 20 mw/mK.
In addition to disuse of materials causing ozone-layer depletion and effective use of resources by recycling or the like, reduction of electric energy consumption is also required for heat/cold retaining apparatuses such as a refrigerator, etc. The aforementioned rigid polyurethane foam which uses an existing blowing agent free from ozone-layer depletion has however a limit in its heat insulating property. Therefore, a technique has been newly proposed for applying a vacuum heat-insulating panel that has heat insulating property twice or more as much as that of the conventional rigid polyurethane foam.
FIG. 11 is a diagram of comparison of heat insulating materials. It is apparent from FIG. 11 that the heat insulating property of the vacuum heat-insulating panel (VIP) is twice or more as much as that in the case of use of HCFC141b-foaming rigid urethane foam (PUF/-141b), cyclopentan-foaming rigid polyurethane foam (PUF/-C-Pentane) and hydrofluorocarbon-foaming rigid polyurethane foam (PUF/-HFC).
For example, JP-A-60-243471 has proposed a heat insulating box having a board disposed in its wall, the board being made by putting pulverized rigid polyurethane foam (PUF) into a synthetic resin bag and vacuum-packing it in a form of a board, and JP-A-60-60483 has proposed a setting method of a vacuum heat-insulating panel in which a gap permitting PUF to flow is provided in the flange side of a side plate.
Each of the proposed vacuum heat-insulating panels and other general vacuum heat-insulating panels is shaped like a board having a thickness in a range from 10 to 20 mm. These panels are installed in walls of a refrigerator.
For production of a refrigerator having a vacuum heat-insulating panel installed therein, an inner box is inserted into an outer box having a vacuum heat-insulating panel stuck thereon so as to assemble a box structure together with other members. Next, rigid polyurethane foam is injected between the outer and inner boxes to thereby form a heat insulating layer, and then interior parts and refrigerant circuit parts are attached to the box structure to complete a product.
Accordingly, a core material for a heat-insulating panel needs basic properties of: having a predetermined strength for holding the shape of the panel in a vacuum state; and suppressing the quantity of heat conducted through a substance constituting the core material (heat conduction) and the quantity of heat transmitted through the substance (heat radiation).
As proposals to obtain the aforementioned properties, for example, JP-A-60-205164 teaches application of rigid urethane foam comprising open cells communicated with each other, as a core material of a vacuum heat-insulating panel, JP-A-60-71881 teaches application of pearlite powder as a core material of a vacuum heat-insulating panel, JP-A-4-218540 teaches application of a plate-like molded product obtained by sintering thermoplastic urethane resin powder in a mold as a core material of a vacuum heat-insulating panel, and JP-A-7-96580 teaches application of a board comprising long glass fiber, fibrillated resin fiber and inorganic fine powder, as a core material of a vacuum heat-insulating panel. Among those materials, communicated open-cell rigid urethane foam, light in weight, excellent in mass-productivity, and excellent in handling property at the time of production, for example, at the time of inserting a core material into a packaging material of a vacuum heat-insulating panel, has been used as a core material.
Further, for improving heat insulating property, such heat insulating mechanisms are effective so that: a substance of low heat conduction is used for constituent materials, contact areas between the materials are reduced to thereby reduce the heat conduction area; heat conduction is controlled to be in a planar direction perpendicular to the heat insulating direction (the direction of the thickness) to thereby suppress the heat conduction through a substance in the heat insulating direction; and a substance having high ability to reflect heat is mixed to thereby reduce heat radiation. Further, to achieve improvement of heat insulating property, JP-A-62-13979 teaches one to bury metal foil or metal vapor deposition film excellent in effect of shielding from heat radiation, and JP-A-63-135694 teaches one to use PUF mixed with fine powder such as calcium silicate or the like.
In order to mix a granular substance of calcium silicate or the like as a filler, for the purpose of suppressing heat radiation, a considerably large quantity of the granular substance is required. Accordingly, weight increases. Further, since the heat conductivity of such a filler is high, heat insulating property is not improved sufficiently. Further, even in a core material containing metal foil disposed therein, heat conduction is not attenuated though it is limited only in the planar direction. Accordingly, there is no effect for suppressing heat conduction between the constituent materials. Further, if the pulverized rigid polyurethane foam described in JP-A-60-243471 is used as it is, such handling difficulties are caused as difficult insertion of the pulverized rigid polyurethane foam into the vacuum heat-insulating panel, large volume reduction of the pulverized rigid polyurethane foam in a packaging bag after evacuating the inside of the packaging bag into a vacuum state and so on.